Code-division multiple access (CDMA) is a spread spectrum technology for sharing resources in a cellular telephone system. Within a given cell, multiple users usually share the frequency spectrum. One method of sharing the spectrum is division by frequency, in which the frequency spectrum is divided into multiple frequency channels and each mobile station is allocated a frequency channel to communicate with the base station of the cell. This method is known as frequency division multiple access (FDMA). In another method known as time-division multiple access (TDMA), multiple mobile units share a frequency channel but use it at different period of time. Such a method is illustrated in FIG. 1. As shown in FIG. 1, mobile unit one is permitted to communicate with the base station during time periods A and D. Mobile unit two is permitted to communicate with the base station during time period B and mobile unit three is permitted to communicate with the base station during time period C. In this manner, multiple mobile units in the same cell can share the same frequency channel, without interfering with each other's signals.
Unlike FDMA and TDMA, CDMA allows every mobile unit to communicate with the base station using same frequency channel at the same time. CDMA uses a direct sequence spread spectrum technique, in which the baseband information signal is modulated using a code, known as a chipping code, which has a much higher chip rate than symbol rate of the baseband information signal. The result is that the modulated signal is spread over a wider frequency spectrum. In a CDMA cellular system, each mobile unit uses a different set of chipping codes to communicate with the base station. The chipping codes are chosen so that they have low cross-correlation with each other. That is, the chipping codes are designed for maximum separation from each other. The base station can identify each transmitting mobile unit based on the chipping code used in the transmission. However, with the widespread use and popularity of mobile phones, it has become increasingly desirable to provide mobile phones with high speed internet access, data, video and music services. High data transfer speeds are required to provide such services. Thus, third generation (3G) networks have been developed which are based on a high speed version of CDMA known as wide band CDMA (WCDMA), which uses a wider frequency spectrum than CDMA, thus providing more bandwidth for data transfer.